Multi-carrier/DS-CDMA mobile communications and single carrier/DS-CDMA mobile communications are used as DS-CDMA mobile communications.
When data length to be transmitted is far longer than one packet length in a single carrier/DS-CDMA uplink packet transmission method, an efficient transmission is achieved at the time of call connection by fixedly assigning spreading codes and time slots to be used in communication.
In this case, efficient multiplexing is necessary with respect to reservation demand packets for reserving the allocation of spreading codes and time slots as well as with respect to data packets for transmitting real data. Since the probability of reservation demand packets being able to be transmitted are not controlled in the conventional technique, collision of reservation demand packets occur frequently at the time of heavy traffic, giving rise to a problem that a transmission efficiency decreases.
In “Performance of orthogonal CDMA codes for quasi-synchronous communication systems” (V. DaSilva, E. Sousa: Proc. of ICUPC' vol. 2, pp995-999, 1993), first study was made about the multi-carrier/DS-CDMA method.
Unlike the single carrier/DS-CDMA, which transmits a CDMA signal on one carrier, the multi-carrier/DS-CDMA divides a radio-transmission bandwidth, and performs parallel transmission of a CDMA signal by two or more subcarriers.
With this provision, the speed of information transmission per subcarrier becomes low, and the speed of code spreading at which information signals are spread to generate CDMA signals also becomes low. Consequently, a chip length of the spreading code can be made longer in the multi-carrier/DS-CDMA than in the single carrier/DS-CDMA. As the chip length is increased, the influence of synchronization errors between spreading codes will be alleviated. Utilizing this feature, the above-mentioned paper applies the multi-carrier/DS-CDMA to communications from mobile stations to base stations in mobile communications systems, and presents a method of performing quasi-synchronous transmission.
Further, a performance evaluation has been conducted on a link level of the multi-carrier/DS-CDMA.
“On the Perfomance of Multi-carrier DS CDMA Systems” (S. Kondo and L. B. Milstein: IEEE Transactions on Communications, vol.44, no.2, pp.238-246, February 1996) demonstrates that the multi-carrier/DS-CDMA provides better properties than the single carrier/DS-CDMA according to a performance evaluation that is conducted in the presence of narrow-band interference.
However, conventional studies about the multi-carrier/DS-CDMA method have been centered around performance evaluations on the link level, and few studies have been made on how a mobile station should communicate with a base station and how a control signal therefor should be transmitted when this method is applied to a mobile communication system.
Furthermore, these studies have a problem in that they have been based on the circuit switching method which always secures a communication channel of exclusive use from the start of transmission to its end for signal transmission from a transmitter to a receiver as usually used in the conventional mobile communication system.
Moreover, digital mobile communication systems (for example, a PDC system, a GSM system, and the like) are focused onto voice communication services, and the systems are designed on the basis of the circuit switching at present. Further, although a packet transmission service is planned also in the next-generation mobile communication system (for example, IMT-2000), which will be introduced soon, the system design is also based on the circuit switching. Thus, in a radio-transmission system based on the circuit switching, channels for an uplink and a downlink are almost equal in number, and even if a data communication is performed, a symmetrical channel structure of the uplink and the downlink is used. For this reason, transmission of a control command to each user is performed using the channel corresponding to each user.
Conversely, in multimedia mobile communications for which demand is supposed to grow in the future, communications are asymmetrical between the uplink and the downlink. Accordingly, it is conceivable that downlink traffic accounts for most of the traffic when data is downloaded. Also, when data is uploaded, it is conceivable that the uplink traffic accounts for most of the traffic while the downlink is required only for response signals. In such a case, if one channel assigned to each user is used to send each user's control command, the efficiency of data transmission becomes poor.
In recent years, the multi-carrier/DS-CDMA mobile communication system has been attracting attention as a mobile communication system that is capable of high-speed data transmission. This multi-carrier/DS-CDMA mobile communication system has a problem in that no scheme has yet been proposed about a channel structure that takes into account the above-described asymmetry in the amount of data transmission between the uplink and the downlink in high-speed data communications directed to multimedia or the like.